I. Field of the Invention
The present invention relates generally to wireless radio telephone systems that use a transmitting station having at least two antennas for transmitting information to one or more receiving units served by the transmitting station. More particularly, the present invention relates to a novel and improved method and apparatus for providing diversity transmissions to a receiving unit served by the transmitting station.
II. Description of the Related Art
It is well known in the art that the performance of the reverse link of a wireless communications system can be improved by providing multiple transmission paths. With multiple transmissions paths, a diversity gain is obtained by combining the phase and amplitude adjusted signals from the different paths when they are received. The combined signal is less likely to fade to an unacceptable level because deep fades in the combined signal occur only when the fades in the individual paths are aligned with each other.
It is also possible to obtain signal diversity in the channel from the base to the mobile, i.e. in the forward link. However, obtaining signal diversity in the forward link is more difficult than obtaining it in the reverse link because mobile units must be as small and simple as possible. Dual mobile receive antennas are commonly used in a mobile unit in some parts of the world. For example, in Japan, it is common to use dual receive antennas with the PDC (Personal Digital Cellular) system. Most of these antennas are switched (i.e., the signal is not received simultaneously by both antennas). This has the advantage of sharing the front end hardware in the mobile station, but does not result in as good a reception as the case in which both antennas are receiving simultaneously. However, this approach increases the cost of each of the more numerous mobile receivers. There are several ways known in the prior art to obtain transmit diversity on the forward link with only one mobile receive antenna.
One consideration in providing transmit diversity in a code division multiple access (CDMA) communication link is the fact that each of the transmit channels are transmitted orthogonally to each other. One such CDMA system is described in Electronics Industry Association standard TIA/EIA/IS-95-A, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”. By transmitting signals that are orthogonal to each other, the capacity of the system is significantly increased as the interference between channels is reduced between channels. In the TIA/EIA/IS-95-A CDMA system, each channel is distinguished from each other channel by an orthogonal Walsh code. For these codes to remain totally orthogonal at the receiver, there cannot be any “multipath” (i.e., more than one path between the transmitter and receiver) on the forward link. When there is a multipath, multiple signal “components”, each having different delays, are received. Each such component comprises a number of channels. The channels within a particular multipath component are orthogonal to each other, but are not orthogonal to the channels in a different multipath component assuming that the amount of path delay between transmission and reception differs for each such component due to these differences.
In one prior art method, multiple delayed versions of the forward link signal can be transmitted. Different forward link signals are by transmitted different separate antennas. This provides multipath which provides diversity. If the delayed versions are separated by enough time, the mobile station's receiver can resolve the different multipaths and obtain the benefit of the diversity (e.g., the transmissions from each antenna will typically not fade together). However, this method has the disadvantage that in a CDMA system, the multiple paths interfere with each other, since they are not orthogonal due to a misalignment of the Walsh codes of the undelayed signal with respect to the delayed signal. Even when the base to mobile link only produces one resolvable path, the additional paths from the signals radiated by the other antennas interfere with the original signal. Similarly, the original signal will not be orthogonal to the delayed signal, and so will interfere with the signals radiated by the other antennas.
In another method, the forward link signal can be transmitted from multiple base station antennas using a different carrier frequency for each antenna. In this case, the same modulation symbols can be sent on different frequencies. Alternatively, error correcting coding may be used and the code symbols can be sent on different frequencies. Several combinations of these methods can be used. Since the signals are on different carrier frequencies, the received signals at the mobile do not interfere with one another. Additionally, different frequencies have different fading characteristics, especially if the frequencies are widely separated. This helps to provide different diversity paths. The forward signal can also be transmitted on multiple base station antennas with the same carrier frequency using orthogonal waveforms. For example, in a CDMA system different Walsh codes can be used on the multiple antennas. Code symbols for any user can be transmitted using one Walsh code on one antenna and another Walsh code on another antenna. If the transmit antennas are close to each other and the signals transmitted over the primary paths received from the multiple base station antennas are time aligned, the signals remain orthogonal. Thus, with small primary path time differences, a receiver for a finger of the rake does not receive interference from the other primary paths. However, when there is more than one path, each with different delays from a base station to the mobile, the paths will not be received as orthogonal to one another.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.